How does a magnetically levitated transit vehicle work? — LB, West Palm Beach, FL
Although there are a variety of schemes for magnetically levitating trains, perhaps the most promising is a technique called electrodynamic levitation. In this scheme, the train contains very strong magnets (probably superconducting magnets like those used in MRI medical imaging systems) and it travels along an aluminum track. The train starts out rolling forward on wheels but as its speed increases, the track begins to become magnetic. That’s because whenever a magnet moves past a conducting surface, electric currents begin to flow in that surface and electric currents are magnetic. Thus the moving magnetic train makes the aluminum track magnetic. For complicated reasons having to do with electromagnetic induction, the track’s magnetic poles are oriented so that they repel the magnetic poles of the train—the two push apart. While the track can’t move, the train can and it floats upward as much as 25 cm (10 inches) above the track. Once the magnetic forces can support the train, the wheels are retracted and the train floats forward on its magnetic cushion. To keep the train moving forward against air resistance (and a small magnetic drag force), there is also a linear electric motor built into the train and track. This motor uses additional electromagnets in the train and track to push and pull on one another and to propel the train forward (or backward during braking).